PYROMETRY
449
through the middle. One-half of the field receives light from the furnace and the other half from the ground-glass screen D which is illuminated by the electric lamp F through the condenser lens E. The ocular containing the red-glass screen A and lens B is focused on the dividing edge of this photometric field. G and H are diaphragms which limit the cone of rays employed. The two fields are matched by turning a thumb screw which moves the black-glass wedge W across the path of the light from the furnace. By a system of gears this movement is transferred to a circular scale on the dial K of the instrument. In appearance the pyrometer resembles the Sci-matco, and is used in the same manner. For a normal point setting the pointer is adjusted to read the normal angle and after removing the tube carrying the lens L the instrument is clamped in its case. The flame gage of the amyl-acetate lamp is so mounted that its ground-glass window is adjacent to the diaphragm H. A table is furnished with the instrument giving the relation between the scale reading in angular degrees and degrees of temperature. This instrument is so designed that the object sighted upon is clearly imaged, a distinct advantage over the Wanner pyrometer. The relation between the scale reading a and the absolute temperature & is a. -\- P — Q/& where P and Q are constants determinable by two calibration points. In a recent modification of this instrument, the lamp is mounted in the line of sight, replacing C, and a semi-circular wedge is employed, so that cos a + P
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FIG. 20. — Disappearing filament optical pyrometer.
Morse, Holborn-Kurlbaum, and Leeds & Northrup Optical Pyrometers — the filament of a small electric lamp Fy Fig. 20, is placed at the focal point of an objective L and ocular forming an ordinary telescope which superposes upon the lamp the image of the source viewed. Red glass such as Coming " high-transmission red" is mounted at the ocular to produce approximately monochromatic light. In making a setting the current through the lamp is adjusted by means of a rheostat until the tip or some definite part of the filament is of the same brightness as the source viewed. The outline or detail of this section of the filament is then indistinguishable from the surrounding field. The current is read on an ammeter and the corresponding temperature is obtained from a plot or table of current versus temperature. The relation between the current, i, through the lamp and the temperature, <°C., is of the form :
where a, &, c are constants requiring for their determination at least three standardization points.
The lamps should not be operated at temperatures higher than 1,500°C. on account of deterioration of the tungsten filament. If this temperature is not exceeded the calibration of the lamp is good for hundreds of hours of ordinary use. For higher